A multi-particulate formulation of F4 fimbriae was developed for oral vaccination of suckling piglets against enterotoxigenic Escherichia coli infections. A feasibility test showed that incorporation of F4 fimbriae in a disintegrating pellet formulation consisting of 87.5% Pharmatose 200 M, 2.5% Avicel CL 611 and 10% Explotab by extrusion/spheronisation and subsequent fluid bed drying resulted in the maintenance of 69+/-12% of the biological activity. But subsequent coating resulted in pellets with poor enteric properties, although good in vivo immunising results were obtained after administration to piglets. From the economical point of view, a pellet formulation was optimised to decrease vaccine dose and dosing frequency. After disintegration testing, pellets consisting of lactose (alpha-lactose monohydrate 90 mesh/beta-lactose 75/25 (w/w)) and microcrystalline cellulose in a ratio of 80/20 (w/w) showed a sponge-like structure from which F4 fimbriae could be released. Coating of these pellets resulted in good enteric properties. To improve disintegrating properties of the pellets, the lactose concentration was increased or sodium carboxymethyl starch was added. But this resulted in poor enteric properties after coating. Dissolution test showed that F4 fimbriae were released from the optimised enteric-coated pellets but interaction between F4 fimbriae and the coating polymer was seen. This incompatibility leads to unpredictable in vitro quantification of F4 biological activity.